The present invention relates to an optical demultiplex circuit for demultiplexing signal light multiplexed at a small wavelength interval.
In a narrow-band multiwavelength multiplex transmission system that constructs a large-capacity transmission system, an optical demultiplex circuit for demultiplexing signal light multiplexed at a small wavelength interval is essential. Such an optical demultiplex circuit is formed by combining an AWG (Arrayed Waveguide Grating), dielectric multilayered filter, fiber Bragg grating, chirped fiber grating, and optical circulator.
However, an AWG is generally expensive, has a large insertion loss of about 6 dB, and requires a temperature control circuit. A dielectric multilayered filter or a chirped fiber grating has large side lobes and, generally, dependency on temperature. In addition, when an optical circulator and a fiber Bragg grating are combined, the arrangement for demultiplexing a number of wavelengths becomes complex.
Furthermore, in use of an output from a pumping laser source used for an optical amplifier using a fiber doped with rare earth elements, the upper limit of the number of wavelengths is about 30 to obtain a gain of 25 dB. To solve this problem, polarized wave synthesis or wavelength multiplex of the pumping laser source, and use of a solid-state laser source have been proposed. However, these methods make the apparatus very expensive and bulky.